High temperature low friction surface coating

ABSTRACT

A high temperature, low friction, flexible coating for metal surfaces which are subject to rubbing contact includes a mixture of three parts graphite and one part cadmium oxide, ball milled in water for four hours, then mixed with thirty percent by weight of sodium silicate in water solution and a few drops of wetting agent. The mixture is sprayed 12-15 microns thick onto an electro-etched metal surface and air dried for thirty minutes, then baked for two hours at 65° C. to remove the water and wetting agent, and baked for an additional eight hours at about 150° C. to produce the optimum bond with the metal surface. The coating is afterwards burnished to a thickness of about 7-10 microns.

The Government has rights to this invention pursuant to Contract No.NAS-3-19427 awarded by the U.S. Department of Energy.

BACKGROUND OF THE INVENTION

Recent and developing applications for compliant hydrodynamic gasbearings require operation at high rotational speeds, extremely hightemperatures, under heavy loads and in the absence of oil lubrication.The gas turbine is a prime example of this application. Uses of the gasturbine are expanding rapidly and include prime mover and auxiliarypower systems for aviation, marine and automobile applications. Thecompliant hydrodynamic gas bearing offers potentially higher operatingtemperatures, elimination of oil lubrication requirements andlimitations, greater accomodation of thermal distortion, assemblyvariations, tolerance of dynamic shaft motion because of bearingcompliance, reduced frictional power loss, reduced rotor noise and lowerbearing costs. Accordingly, this bearing is ideal for gas turbineapplications.

One troublesome problem that has delayed full implementation of thecompliant hydrodynamic gas bearing has been damage to the bearingsurface during start and stop phases of operation. The supporting gasfilm generated in a hydrodynamic bearing due to relative rotation of thebearing surfaces is not sufficient to support the rotor until it reachesa certain speed. At that time, the gas film lifts the rotor from thebearing surface and thereafter the gas lubrication prevents furthercontact between bearing surfaces. Although the relative rotation duringthe contact phase is quite slow, the cumulative effect of the contactcan be sufficient to gall the bearing surfaces. In addition, shocks orviolent eccentric loading of the rotor can cause momentary contact ofthe bearing surfaces which also can result in galling of the bearingsurfaces. The galling can weaken the flexible bearing sheet and canactually interfere with the operation of the bearing during thehydrodynamic phase of the operation.

One solution which has been attempted in the past is to provide a lowfriction coating on the bearing surfaces. Low friction coatings appearto be promising, but are not usually adaptable to application incompliant bearings because of the particular requirements. In general,the known prior art coatings have been developed for rigid members,whereas, in compliant bearings, the coating is applied to a flexiblebearing sheet which is continually flexed during operation. As aconsequence, the coating itself must be flexible or it will crack andpossibly break up during operation. If this happens, the bearing surfacecan become damaged very quickly and, even worse, the abrasive wearproducts of the crumbling coating can themselves greatly shorten thelife of the bearing. To achieve flexibility, the coating must be appliedin a very thin layer, but a very thin layer will wear through rapidly ifit is not extremely tough. In addition, the coating material must bondpermanently to the substrate and be unaffected by flexing, temperaturechanges and harsh environment agents. Finally, the coating must performwell at ambient or start-up temperature, at the normal operatingtemperature of the machine on which it is installed, such as a gasturbine, and at all intervening temperatures. For example, when theengine is started, it can be quite cold. When it is stopped, thetemperature of the bearing is normally at the operating temperature ofthe engine which can be in the neighborhood of 430° C. Coatings whichfunction well at lower temperatures are often found to deteriorate athigher temperatures. Some high temperature coatings which function wellat elevated temperatures do not provide good anti-frictioncharacteristics at lower temperatures.

SUMMARY OF THE INVENTION

It is thus an object of my invention to provide a low friction coatingfor metal surfaces of a flexible substrate operable over a widetemperature range. The coating must be extremely durable to provide longlife in hostile environments such as high temperature and in thepresence of gas lubricants, such as air, hydrogen, argon and helium. Inaddition, a method must be available for applying the coating to a thinflexible metal substrate. The method must be extremely reliable sincefaults in the coating would be difficult to detect and could result inpremature failure of high cost, sophisticated equipment. The method andthe materials used in producing the coating should be inexpensive andreadily available. Finally, they should not produce wear products thatwould interfere with the operation of the bearing or other areas of themachine or otherwise be incompatible with the operation of the machine.

One embodiment of the invention which meets all of the above objects andothers is made by mixing cadmium oxide and graphite powders inproportions of about 1 to 3 and agitating the mixture in water.Approximately 28-32% by weight of sodium silicate in water solution andone drop of wetting agent in proportions of one drop to 250 cc ofsolution are added and stirred vigorously then sprayed onto a carefullypretreated substrate, in a coating about 12-15 microns thick. Thecoating is baked in an oven at 65° C. for two hours, and afterwards isbaked at 150° C. for eight hours. The baked coating is then burnished toabout 7-10 microns thickness. This provides a flexible coating for aflexible substrate which is chemically stable at elevated temperatures,is dimensionally stable and compatible thermally and chemically with theflexible substrate. The coating is resistant to thermal and mechanicalshocks and provides protection for the substrate against corrosion. Itis highly conductive thermally to rapidly dissipate frictional heat fromthe bearing interface, and is easily and reliably applied to thesubstrate with a secure bond in a coating that is uniform and free offaults. It is a low friction and durable coating that provides goodantigalling and self-healing characteristics in the event that thecoating is penetrated, and it is available at acceptable cost.

DESCRIPTION OF THE DRAWINGS

The invention and its foregoing objects and advantages will be betterunderstood by reference to the following detailed description of thepreferred embodiment when read in conjunction with the followingdrawings which is a perspective view of a compliant fluid hydrodynamicbearing having a flexible bearing sheet on which the coating of thisinvention is applied.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawing, a compliant hydrodynamic fluid journalbearing is shown having a bearing sleeve 10 with a cylindrical bore 12in which is disposed a rotatable shaft 14 supported by a bearingassembly 16. The shaft diameter is actually nearly equal to the innerdiameter of the bearing assembly 16, but is illustrated much smaller forclarity of illustration. The bearing assembly 16 includes a flexiblebearing sheet 18 supported by a resilient supporting element 20. Thebearing sheet 18 is welded as by spot welding to the top of a spacerblock 22 and the end of the resilient supporting element 20 is welded tothe bottom of the spacer block 22. The spacer block end of the assemblyis fixed in the bearing sleeve 10 against rotation. The shaft 14 rotatesin the direction indicated by the arrow and generates a hydrodynamic gaswedge between the surface of the shaft 14 and the bearing surface of thesheet 18 to support the shaft 14 on a film of gas. Advanced bearings ofthis nature are disclosed in application Ser. No. 974,256 of Gray andHeshmat, application Ser. No. 974,262 of Miller and application Ser. No.974,263 of Gray and Bhushan copending herewith.

During the start up and stop phases of operation, the shaft 14 is movingtoo slowly with respect to the bearing sheet 18 to generate sufficientpressure in the hydrodynamic supporting gas film to lift the shaft 14off the bearing sheet 18, and therefore, there is physical contactbetween the surface of the shaft and the bearing surface of the flexiblesheet 18. During these phases of operation, it is necessary that thebearing surfaces be protected from excessive wear and other mechanicaldamage which would adversely affect the operation of the machine ordamage the bearing or the shaft 14. The protection is afforded by anantifriction coating on the shaft 14 and/or the flexible bearing sheet18.

I prefer to coat the bearing sheet 18 with the softer antifrictioncoating. Then, if it wears off after a long period of use, the bearingsheet 18 or the entire stationary bearing assembly can be replaced muchmore easily than replacing the rotor. In addition, using the method ofthis invention, the coating is more easily applied to the bearing sheet18 than to the rotor surface.

The conventional approach to antifriction coatings is to choose one or acombination of two alternative possibilities: hard wear resistantcoatings and soft, low shear-strength coatings. The first approach is toutilize certain naturally occurring oxide films that can provideprotection for metallic surfaces sliding in contact. The other approachis to use certain coating materials that are believed to have a layeredlattice, hexagonal crystal structure which tends to wear smoothly withlittle tendency for surface damage. Also, many combinations of materialscan interact to form eutectic compounds which behave as low shearstrength solid films. In most cases, such desirable interactions arelimited to high temperature or high energy input conditions.

Coating techniques may also affect coating effectiveness; i.e., bondstrength, wear resistance and life. Coating techniques include hardfacing such as thermospray, plasma-spray, detonation gun, welding andbrazing. Another technique is the diffusion process exemplified bynitriding, boriding and carbonizing. Chemical vapor deposition andphysical vapor deposition techniques are also available, includingevaporation, sputtering and ion plating. In addition, there are fusionand electroplating and other proprietary processes which are primarilycombinations of the aforementioned techniques. Therefore, a great manymaterials and a great many techniques of applying the materials areavailable for investigation.

Finally, the addition of binders to the wear resistant material mayimprove the bond and the thermal and mechanical shock resistance of thecoating. Binders may also improve the self-healing characteristics ofthe coating so that mechanical damage to the surface of the coating canbe corrected by the action of the bearing itself to protect thesubstrate from oxidation and prevent the delamination of the coatingfrom the point of mechanical damage.

The emphasis in the prior art has been to provide soft lubricant filmsin relatively low temperature applications; i.e., 315° C. to 370° C. Attemperatures above this, it has previously been necessary to use thehard-to-hard combination of materials because the known softer materialscould not stand the high temperature. The hard-to-hard combinations arenot as desirable as combinations including softer materials because ofthe poorer lubricity of the hard materials. In investigating thesubject, the great number of available materials and coating techniquesmust be investigated in enormous numbers of combinations. The number ofcombinations is even further increased by the fact that it is ofsignificance which bearing surface is to be coated with which material.For example, a particular area on the surface of the shaft 14 "sees" thearea of the contact only once per revolution; however, the area on theflexible bearing sheet 18 which is in contact with the shaft during thecontact phase of operation is in constant contact with it and therefore"sees" the contacting surface of the shaft 14 continuously. Thus, theconditions which the bearing surfaces are subjected to are considerablydifferent. The underlying cause of this difference is thermal in nature.When the journal is in rubbing contact with the bearing sheet, thecontact area on the sheet is fixed and all of the frictional heat isconcentrated in one area, while the contact on the journal iscontinuously changing with the shaft rotation and the temperature riseof the journal surface is much more gradual. These conditions affect thechoice of materials for the two surfaces.

The bearing sheet material is INCONEL X-750, a high temperature alloymade by Huntington Alloys of Huntington, W. Va. It is preferred becauseof its ease of heat treatment, good spring properties, availability inthin foil sections, moderate cost, and because it can tolerate the hightemperature and retain its good physical properties. It is anickel-based alloy containing nickel, chromium, iron and titaniumprimarily. The shaft material is A-286 stainless steel which is commonlyused in high temperature gas turbine applications. This material has thenecessary thermal and mechanical properties at 1200° F. and provides asatisfactory trade off between machinability, availability, cost andhigh temperature strength. It is an iron-based alloy containing theelements of iron, nickel, chromium, titanium and manganese, primarily.

After much investigation, I have found that a mixture of cadmium oxideand graphite, known to be a good lubricant in powder form, can beapplied to a flexible substrate in a ceramic matrix as a flexiblecoating that can withstand constant temperatures of 430° C. and briefexcursions to 540° C. without thermal damage. It provides a durable,long lasting, low friction protection for the flexible bearing sheet ina compliant bearing.

The process of applying this coating to the Inconel X-750 surface ismost effective if preceeded by careful preparation of the flexiblebearing sheet. The bearing sheet is heat treated to give it the desiredproperties of hardness and resilience, but the heat treatment results ina thin, slippery oxide layer on the surface to which bonding of thecoating is difficult to achieve. Therefore, this oxide layer should bestripped off and the surface should be roughened in order to achieve asecure bond for the coating. The valleys on the surface after rougheningprovide anchoring points, and particles of the coating material areinterlocked in these valleys, so the roughening procedure militatessignificantly for the bond strength of the coating. This has beenconfirmed by metallurgical examination. A coating surface which wasburnished to remove all the coating was examined under a scanningelectron microscope, revealing that the coating material particles weretrapped in the grain boundaries of the substrate as desired.

Numerous processes of pretreating the surface to be coated areavailable. These include phosphating, sandblasting, vapor honing, handpolishing, acid etching and electrolytic etching.

I considered the mechanical treatments to be unacceptable because of thedistortion they produce in the thin bearing sheets which range inthickness between 50 and 200 microns, with most between 100 and 150microns. It is important in the operation of compliant hydrodynamicbearings that the bearing sheet lie flat; curling and wrinkling of thebearing sheet produced by these mechanical treatments make it difficultor impossible to ensure the necessary flatness.

Acid etching attacks the surface nonuniformly causing the oxide layer tocome off in some spots earlier than others and for the surface to bepitted. I believe this lessens the mechanical strength and the fatiguelife of the bearing sheet which can be subjected in use to extremelyhigh stresses.

A cleaning and roughening technique, which I found to work well, waselectroetching. The etching solution consists of 25 grams of sodiumfloride and 95 cc sulfuric acid in 1 liter of water. The INCONEL X-750bearing sheet is wired to a 6 volt power supply and inserted as theanode in the etching solution (any other material can serve as thecathode) and the sheet is etched at 2325 amp per square meter for 2minutes. This removes the oxide layer completely and attacks the grainboundaries somewhat primarily removing small amounts of chromium andgamma prime phase. The grain surface is not attacked, so the resultingprofile is a mosaic of grains, perhaps 20 microns in diameter,surrounded by etched lines about 0.28 microns deep at the grainboundaries. The concentration of chromium on the etched surface isfairly uniform, indicating that the loss of chromium at the grainboundaries does not significantly affect the composition of thematerial. Whatever loss of chromium and chromium carbides that may occurat the grain boundaries is not sufficient to affect the oxidationresistance of the material. Concentrations of nickel and titanium arefairly uniform. I believe that the loss of a gamma prime phase, theintermetallic compound Ni₃ (Al, Ti), which is formed in the graininteriors and adjacent to the grain boundaries and is responsible forthe increase in tensile and fatigue strength which occurs during agehardening, is restricted to the boundaries and is so small that itseffect on the tensile and fatigue strength of the material isinsignificant. My tests have confirmed that the etching produces hardlyany change in the strength of the bearing sheet. Indeed, I found thatthe fatigue strength of the bearing sheet etched on one surface isactually slightly higher than that of an unetched sheet. I believe thatthis affect occurs because the etching removes mechanical defects, suchas microcrevices and micronotches from the surface, thus improving thefatigue strength. However, I found that the tensile and fatigue strengthof the material etched on both sides is slightly lower than the materialetched on only one side. Thus, it is advisable to mask one side of thebearing sheet and etch only the side that is to be coated with thecoating material. The masking can be done with stop-off lacquer such asno. 323 red supplied by M & T Chemicals, Inc., N.J. The average or rmsroughness of the etched foil is about 0.28 microns. A range of 0.23-0.33microns rms is about optimum for this material.

The etched material is then thoroughly washed in water in an ultrasoniccleaner to mechanically clean particulate material from the surface andetched grain boundaries, and afterwards is cleaned with alcohol toremove all chemicals from the surfaces.

The coating composition is prepared by mixing one part cadmium oxidepowder with three parts graphite powder in a quantity of distilledwater. The mixture is agitated, as in a ball mill, until the particlesof cadimum oxide and graphite are finely and uniformly dispersedthroughout the water. I find that four hours in the ball mill issufficient time to achieve the desired dispersement, although it may beaccomplished in lesser time with other techniques.

After agitation, the mixture is removed immediately from the ball milland poured into the container of an air brush. This and the next stepsmust be accomplished with dispatch to prevent the cadmium oxideparticles from settling out of the mixture. A drop of wetting agent inproportions of one drop to about 250 cc of solution is added and themixture is stirred and heated to about 65° C. The wetting agent is Absol895 which is a liquid, nonionic surfactant supplied by Surpass ChemicalCompany, Inc., Albany, N.Y. The mixture is heated to 65° C. because thatis the most effective temperature for the action of the wetting agent.After stirring, 28%-32% by weight of solid sodium silicate, dissolved inwater, is added to the solution and it is stirred vigorously. Lesssodium silicate does not give a good bond, and more sodium silicateproduces a coating of insufficient lubricity. I find that 30% is aboutthe best proportion. The viscosity of the mixture at this point isadjusted by adding distilled water so that the mixture is liquid enoughto spray through the air brush, but it is not so fluid that it will runoff the substrate when sprayed.

The mixture is now sprayed onto the clean and roughened surface of thesubstrate to a thickness of about 12-15 microns. To prevent the fluidcoating from being disturbed, I allow it to air dry for about 30 minutesat which time the coating is firm enough to be handled without beingdisturbed. The bearing sheet is baked in an oven at about 65° C. for twohours or as long as necessary to remove the water and wetting agent.Then it is baked at about 150° C. until the sodium silicate is fused. Ihave found that a baking time of about eight hours is sufficient to fusethe sodium silicate, although a shorter time at a higher temperaturemight achieve the same result. The bond of the coating to the substrateis extremely good. I believe the explanation for this excellent bond isthat the cadmium oxide reacts with the sodium silicate to producecadmium silicate and possibly other compounds at the bond interfacewhich improves the bond.

The resulting coating is fairly irregular, so it should be burnishedwith fine emery paper to 7-10 microns thickness. This gives a smoothnessof less than 0.15 microns rms and a coating thickness that is thinenough to be flexible, but thick enough to resist rapid wear. A coatingthicker than 12 microns does not adhere well.

The coating produced by this method is sufficiently flexible to use on aflexible bearing sheet of a compliant hydrodynamic gas bearing andmaintain a smooth, continuous surface without developing cracks or otherbreaks in the surface. The coating is tenaciously adherent and verydurable. It offers superior lubricity at low cost over a temperaturerange of below freezing to 430° C., with brief excursions to 540° C.producing no damage. The method of applying the coating is simple andreliable, repeatably producing a coating without defects which meets allspecifications.

Obviously, numerous modifications and variations of the disclosed methodand coating are possible in view of the teachings herein. Thesemodifications and variations, and the equivalents thereof, are expresslyto be understood to fall within the spirit and scope of my inventionwhich is defined in the following claims, wherein I claim:
 1. In acompliant hydrodynamic gas bearing for high temperature applications, aflexible bearing sheet with one bearing surface, and a relativelyrotating rigid member with an opposing bearing surface, the improvementcomprising:a flexible coating of less than 12 microns thickness on thebearing surface of the flexible bearing sheet, including a layer of CdOand graphite in proportions of about 1:3 by weight, dispersed in a fusedmatrix of 28%-32% by weight sodium silicate.
 2. The bearing defined inclaim 1, wherein said bearing sheet is a high temperature, nickel-basedmaterial, also containing chromium, iron and titanium, between 50 and200 microns thick, which has been heat treated for hardness, resilienceand strength, and has been electroetched on said bearing surface toremove the oxide film resulting from said heat treatment and to roughensaid surface, primarily at the grain boundaries, to improve the bond ofsaid coating on said surface.
 3. The bearing defined in claim 2, whereinthe surface roughness of said bearing sheet under said coating isapproximately 0.23-0.33 microns rms.
 4. In a hydrodynamic gas bearing,for high temperature applications, including a stationary member and arelatively rotating member, each member having a bearing surface facingthe bearing surface of the other member, the improvement comprising:ahard oxide or carbide coating on the bearing surface of said rotatingmember; and a solid coating bonded to the bearing surface of saidstationary member, including a dispersion of CdO and graphite powders,in proportions of about 1:3 by weight, in a fused matrix of about28%-32% by weight of sodium silicate.